This invention relates generally to apparatus and methods for the generation of singlet oxygen atoms which may combine with oxygen molecules to produce ozone. For the purpose of describing this invention, the term "ozone" includes both singlet oxygen and O.sub.3 molecules. In particular, the invention is directed to apparatus in which a high energy electron beam generated by a hollow cathode plasma discharge device is directed into an oxygen-containing atmosphere moving at a high velocity to produce substantial quantities of ozone at high efficiency and to methods of ozone generation using electron beams. The invention is particularly adapted for use in sewage treatment, odor and color removal, water and air purification, the removal of hydrogen sulfide from stack gases, and other instances in which environmental wastes may be removed by oxidation with ozone, as well as in oxidation of chemicals used in maufacturing or produced by manufacturing processes.
Methods of treating waste water, sewage, drinking water, environmental air and industrial stack gases for disinfection and for removing impurities by oxidation have long been known. Chlorine, the most prevalent oxidizing disinfectant, causes oxidation in water by forming hypochlorous acid, HOCl, in which the singlet oxygen atom is the oxidizing agent. Another source of singlet, or nascent, oxygen is ozone, which can be made in unlimited supply from air and which does not produce long-lasting toxic by-products such as the carcinogenic chloramines produced by chlorine.
In the past, the most efficient method for generating commercial quantities of ozone used silent electrical discharge, but this method has a number of disadvantages which result in relatively low ozone generation efficiency. One such disadvantage is the requirement that substantially dry air or oxygen be used. The silent discharge method theoretically produces ozone from pure O.sub.2 at yields up to about 150 g/kwh, or about 3 kwh consumed for each pound of ozone molecules produced. The optimum yields from air are considerably less, being about 12 kwh/lb O.sub.3. These disadvantages make the use of the silent discharge method for the generation of ozone in situ at water purification plants and in stack gas scrubbers unsuitable for large scale use at a cost competitive with chlorine.
The use of electron beams directed into oxygen to produce singlet oxygen atoms which combine with O.sub.2 molecules to produce ozone has been proposed in U.S. Pat. Nos. 3,883,413 and 3,702,973. These patents disclose the use of thermionic electron guns to produce beams of electrons which collide with oxygen molecules, dissociating them and forming secondary electrons and ion pairs. The efficiency of ozone generation disclosed in U.S. Pat. No. 3,883,413, namely 3 kwh/lb O.sub.3, represents only an ideal design parameter which is markedly inferior to the actual efficiencies observed in apparatus according to our invention. Thermionic electron guns, however, require maintenance of very low pressure around the thermionic emitter, on the order of 0.001 micron, and are readily susceptible to cathode poisoning. The pumping required to maintain this very low pressure reduces the overall efficiency of the thermionic gun when adapted for use in an ozone generator.
Recently, the hollow cathode plasma discharge device has been available as a source of high energy electron beams. Hollow cathode plasma discharge devices (HCD) are disclosed in U.S. Pat. Nos. 3,831,052 and 3,970,892. Our invention employs a HCD in which the electron beam is intentionally spread out to cover the window separating the vacuum enclosure of the HCD from the atmosphere. Our invention does not require the uniformity in the electron beam required in the laser keying and electron irradiation uses disclosed in U.S. Pat. Nos. 3,831,052 and 3,970,892. The HCD is particularly advantageous as a source of electrons for field use, as it is relatively small in size, does not use delicate filaments for generating electrons, and is simple of construction with considerable ease of maintenance.